1. Field of the Invention
This invention relates to bituminous concrete and the methods of making same, and more specifically to sulfur-extended asphalt mix designs, processes, and apparatus for use with the same.
2. Description of Prior Art
Preliminarily, as used herein, the terms "asphalt" or "asphalt cement" shall mean any of the heavy petroleum oils or tar or pitch; "bituminous contrete" shall mean a composition of asphalt cement and aggregate (such as gravel, sand, mineral fillers, etc.); and "sulfer-extended asphalt" shall mean a mixture of sulfur and asphalt cement. Additionally, the term "binder content" shall mean, depending upon the context, either the weight of the asphalt cement alone or the weight of the sulfur-extended asphalt mixture, expressed as a weight percentage of the total weight of the bituminous concrete mixture. The units of measurement for the results of the well-known Marshall Flow test noted herein are in one one-hundreths of an inch. For example, if a bituminous concrete test specimen deforms 0.15 inch, the Marshall Flow value is noted as 15.
There have been various prior attempts to eliminate dependence upon petroleum in the manufacture of compacted bituminous concrete or so-called blacktop by using sulfur-extended asphalt compositions. The majority of such teachings require that molten sulfur be utilized in making the sulfur-extended asphalt. However, such prior art liquid sulfur-extended asphalt methods require costly and specialized equipment to maintain the elevated temperature of the molten sulfur (at 246.degree. F. or more) and also to properly blend it with the liquid asphalt cement. In a typical asphalt batch plant, the use of liquid sulfur would require heated liquid sulfur storage tanks, supplemental burners, and associated heated valves, pumps and piping equipment. Additionally, since the molten sulfur and asphalt are preblended before being introduced into the asphalt weigh bucket and finally into the the heated aggregate to form the bituminous concrete, costly high shear energy-type blending units must be used due to liquid sulfur's relatively high viscosity. These include colloid mills, gear pumps, high speed stirrers, propeller mixers, static mixers, or other such devices.
Additionally, the known prior art mix designs for liquid sulfur-extended asphalt typically require that the asphalt be replaced by liquid sulfur on an equal volume basis. This means then that, since sulfur is twice the weight of asphalt, two weight units of sulfur must be included for every asphalt unit being replaced. Such a 2:1 weight ratio replacement was primarily used to minimize the high air void contents found in many sulfur-extended bituminous concrete mixtures of the prior art. Due to the typically prevailing market prices for liquid sulfur and asphalt cement, such prior art liquid sulfur-extended mix designs were not satisfactory from a cost savings standpoint. Thus, they were not widely utilized. The latter is also true because the supply of liquid sulfur has not historically been reliable. Consequently, until the present invention the manufacture of bituminous concrete has remained highly petroleum dependent. Moreover, from an energy consumption standpoint, the extra energy required to maintain molten sulfur at useable liquid states with such prior art mix designs is highly disadvantageous.
There have been prior art attempts to replace liquid asphalt cement by solid sulfur. Such prior art mix designs were also typically produced on an equal volume basis, i.e., a sulfur-to-asphalt weight ratio of 1.75:1, 2:1, or even higher. At such high sulfur concentrations, the resulting pavement product was difficult to compact using conventional rolling techniques. Thus, such prior art designs that were used in relation to highway paving were primarily for repair work. That is, individual segments of asphalt were cast without subsequently being compacted, such as for repairing potholes, for example.
Examples typifying such prior art sulfur-extended asphalt compositions for use in bituminous concrete are disclosed in U.S. Pat. Nos. 2,182,837, 3,738,853, and 3,960,583, British Pat. No. 1,363,706, and Canadian Pat. Nos. 755,999, 945,416 and 1,042,610.